Calcium fluoride optics with improved laser durability

ABSTRACT

The invention is directed to calcium fluoride crystal optics with improved laser durability that can be used for the transmission of below 250 nanometer (nm) electromagnetic radiation. The optics consist of CaF 2  as the major component and, in one embodiment, at least one dopant/amount selected &gt;0.3-1200 ppm Mg, &gt;0.3-200 ppm Sr, &gt;0.3-200 ppm Ba, while Ce and Mn are &lt;0.5 ppm. The doped crystal and optics made therefrom have a ratio of 515/380 nm transmission loss of less than 0.3 after exposure to greater than 2.8 MRads of γ-radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Application Ser. No. 61/11,0195 filed on Oct. 31,2008.

FIELD

The invention is directed to calcium fluoride crystals and optics madetherefrom with improved laser durability that can be used for thetransmission of below 250 nanometer (nm) electromagnetic radiation.

BACKGROUND

Excimer lasers are the illumination sources of choice for themicrolithographic industry. The use of high power lasers, for example,those with pulse energy densities (fluence) above 20 mJ/cm², with pulsewavelengths below 250 nm (for example, 193 nm and below) can degrade theoptics used in laser lithography systems. T. M. Stephen et al., in theirarticle “Degradation of Vacuum Exposed SiO2 Laser Windows,” SPIE Vol.1848, pp. 106-109 (1992), report on the surface degradation of fusedsilica in an Ar-ion laser. More recently, it has been noticed that thereis optical degradation in high peak and average power 193 nm excimerlasers using materials made from substances other than silica.

Ionic materials such as crystals of MgF₂, BaF₂ and CaF₂ are thematerials of choice for excimer optical components due to theirultraviolet transparencies and their large band gap energies. Of thesethree materials, CaF₂ is the preferred material due to its cubic crystalstructure, performance, quality, cost, and relative abundance. However,the polished but uncoated surfaces of CaF₂ optics are susceptible todegradation when exposed to powerful excimer lasers operating in thedeep ultraviolet (“DUV”) range, for example at 248 and 193 nm and thevacuum ultraviolet (“VUV”) range, for example at 157 nm. For lasersoperating at 193 nm, 2-9 KHz, with pulse energy densities of 20-80mJ/cm², the surfaces of the optical elements made from these ionicmaterials are known to fail after only a few million laser pulses. Inother applications, for example medical lasers, alternate operatingparameters could exist such as 193 nm laser fluences of 200 mJ/cm²-1000mJ/cm² (very high fluences) and very low repetition rate (for example10-100 Hz) that may also result in the accelerated failure of suchoptical elements. The laser damage is thought to be the result offluorine migration from the crystal optic interior or bulk to thesurface where the fluorine is lost to the atmosphere. The loss offluorine from the CaF₂ crystal optic results in the formation of Fcenters which can then combine to form Ca colloids near the surface andwithin the bulk. These Ca colloids subsequently increase scatter andheating of the optical element, with eventual catastrophic failure. U.S.Pat. No. 6,466,365 (the “365 patent) describes a method of protectingmetal fluoride surfaces, such as of CaF₂ optics, from surfacedegradation by use of a vacuum deposited coating, for example, a siliconoxyfluoride material. While coatings may be sufficient to addresssurface damage, the microlithographic industry constantly demandsgreater performance from excimer sources, and consequently from opticalcomponents used in connection with excimer laser based systems.Therefore, the laser durability of the bulk material, CaF₂, must also beimproved by limiting the formation of Ca colloids that result in theeventual failure of the optical element. This solution will eithereliminate the problem or greatly extend the bulk durability andconsequently the length of time that existing and future opticalelements can be used without having to be replaced.

Solutions to the issue of optical element lifetime involving the use ofother optical materials, such as MgF₂, have been considered. However, itis believed that such materials will also experience degradation similarto that of CaF₂ with time, leading to the same requirement; i.e. thatthe expensive windows be replaced. It is further believed that thedegradation problems of CaF₂, MgF₂, and other fluoride optical materialswill be exacerbated with the advent of laser systems operating atwavelengths below 193 nm. Thus, identifying a method to increase thelaser durability of the CaF₂ bulk appears to be the most straightforwardmethod of achieving the industry demands for improved laser performance.

SUMMARY

In one aspect the invention is directed to doped CaF₂ crystals, andoptics made therefrom, that can be used in below 250 nm laser systems,including laser microlithographic systems. The optics are made fromcrystal CaF₂ material that has been doped with a selected amount ofdopant material, for example without limitation, magnesium (Mg). In aone embodiment the amount of dopant is less than 2500 ppm. In anotherembodiment the amount of dopant is >0 and ≦1200 ppm. In a furtherembodiment the amount of dopant is >0 and ≦500 ppm. In yet anotherembodiment the amount of dopant is >0 and ≦200 ppm.

In one aspect, the invention is directed to a laser optic havingimproved laser durability, said optic comprising a CaF₂ crystal materialdoped with a selected amount of a selected dopant, and said optic havinga ratio of 515/380 nm transmission loss of less than 0.3 after exposureto greater than 2.8 MRads of γ-radiation. In one embodiment the dopantand amount is selected from the group consisting of >0.3-1200 ppmMg, >0.3-200 ppm Sr, >0.3-200 ppm Ba. In another embodiment the dopantsare selected from the group consisting of Ce and Mn in an amount of lessthan <0.5 ppm of the selected dopant. In a further embodiment the dopantand amount is 2-500 ppm Mg. In a different embodiment the dopant andamount is 10-100 ppm Mg. In an additional embodiment the ratio of515/380 nm transmission loss is less than 0.2 after exposure to greaterthan 2.8 MRads of γ-radiation. In a further embodiment the ratio of515/380 nm transmission loss of less or equal to 0.1 after exposure togreater than 2.8 MRads of γ-radiation. The laser optic can also have acoating thereon, the coating being at least one material selected thegroup consisting of SiO₂.F, Al₂O₃, MgF₂, BaF₂, CaF₂, SrF₂, NaF, LiF,AlF₃, LaF₃, GdF₃, NdF₃, DyF₃, YF₃ and ScF₃.

In another embodiment the invention is directed to a laser optic havingimproved laser durability, the optic comprising a CaF₂ single crystalmaterial doped with 20-100 ppm Mg, and optic having a ratio of 515/380nm transmission loss of less than or equal to 0.2 after exposure togreater than 2.8 MRads of γ-radiation. In one embodiment the ratio of515/380 nm transmission loss is less than or equal to 0.1 after exposureto greater than 2.8 MRads of γ-radiation. In a further embodiment theoptic has a coating thereon, said coating being at least one materialselected the group consisting of SiO₂.F, Al₂O₃, MgF₂, BaF₂, CaF₂, SrF₂,NaF, LiF, AlF₃, LaF₃, GdF₃, NdF₃, DyF₃, YF₃ and ScF₃.

The invention is also directed to a doped CaF₂ crystal suitable formaking laser optics having improved laser durability, said crystalconsisting of CaF₂ as the major component, and at least one dopantselected from the group consisting of the group consisting of >0.3-1200ppm Mg, >0.3-200 ppm Sr, >0.3-200 ppm Ba. In one embodiment the dopantand amount is 2-500 ppm Mg. In another embodiment the dopant and amountis 10-100 ppm Mg.

In another embodiment the crystal has a ratio of 515/380 nm transmissionloss of less than 0.3 after exposure to greater than 2.8 MRads ofγ-radiation. In an additional embodiment the crystal has a ratio of515/380 nm transmission loss of less than 0.2 after exposure to greaterthan 2.8 MRads of γ-radiation. In a further embodiment the crystal has aratio of 515/380 nm transmission loss of less than or equal to 0.1 afterexposure to greater than 2.8 MRads of γ-radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) illustrates a crystal growth crucible having a seedcrystal reservoir and the axial orientation direction of the seedcrystal.

FIG. 2 (prior art) illustrates the growth crucible of FIG. 1 loaded withdoped CaF₂ feedstock.

FIG. 3 illustrates the crucible of FIG. 2 contained within the upperzone of a two zone furnace, the feedstock and the upper part of the partof the seed crystal having been melted.

FIG. 4 illustrates the change in the ratio of 515/380 nm transmissionloss for un-doped and Mg-doped CaF₂.

FIG. 5 is the Raman spectrum illustrating the formation of colloids inun-doped CaF₂ crystal.

DETAILED DESCRIPTION

As used herein the terms ‘calcium fluoride crystal” and “calciumfluoride optic” means a calcium fluoride crystal, or optic madetherefrom, containing at least one dopant as specified herein and in anamount within the range given for each dopant as specified herein. Thecrystal can be a single crystal such as is grown by the Bridgman method,the Bridgman-Stockbarger method and other methods known in the art, orit can be a crystal formed by heating a calcium fluoride powder orplurality of small crystals under pressure at a temperature such thatthe powder or plurality of crystals fuse to form a calcium fluoridecrystal as is also known in the art. These processes are typicallyconducted under vacuum, in an inert or fluorinating atmosphere, or underconditions containing only minor amounts of oxygen. Examples of crystalsof alkaline earth metal fluorides grown using the Bridgman,Bridgman-Stockbarger, and Czochralski methods, or variations thereof,can be found in, for example without limitation, U.S. Pat. Nos.7,033,433, 6,989,060, 6,929,694, 6,702,891, 6,704,159, 6,806,039,6,309,461 and 6,123,764. The crystals can be made into optics by methodswell known in the art.

As used herein the terms “calcium fluoride single crystal”, “calciumfluoride single crystal optic”, and similar terms including the word“doped”, mean a single crystal of calcium fluoride, or optic madetherefrom, containing at least one dopant as specified herein and in anamount within the range given for each dopant as described herein.Dopant amounts are given in parts-per-million (ppm) by weight of thedopant metal ion in the crystal.

Further, it is to be recognized that the CaF₂ crystals can contain, inaddition to the intentional metal dopants described herein, very lowlevels of other “contaminants”, for example without limitation,contaminants such as those specified herein. All such contaminants areto be deemed as due to the inability to absolutely eliminate suchmaterials from the feedstock or processing environment, and are not tobe deemed as being intentionally present or affecting the durability ofthe doped CaF₂ crystals and optics of the invention. In the art citedabove for making CaF₂ crystals it was preferred that the doped calciumfluoride feedstock is such that the final crystal optic product hasimpurity levels, by weight as measured by ion-coupled plasma massspectroscopy (ICP-MS) or other appropriate method known in the art, ofless than 0.1 ppm Li, less than 4 ppm Na, less than 3 ppm K, less than0.2 ppm Sc, less than 0.2 ppm Y, less than 0.2 ppm La, less than orequal to 0.2 ppm Gd, less than 0.2 ppm Yb, less than 0.2 ppm Ti, lessthan 0.1 ppm Cr, less than 0.5 ppm Mn, less than 0.4 ppm Fe, less than0.2 ppm Co, less than 0.2 ppm Ni, and less than or equal to 0.3 ppm Cu.Preferably the calcium fluoride raw material has less than or equal to0.5 ppm Na and 0.5 ppm K. The total of such contaminants is generallyless than 50 ppm.

The dopants can be added to the CaF₂ feedstock used to make the CaF₂crystal as a fluoride, oxide, carbonate, or finely powdered metal. Themixture of CaF₂ powder and dopant is treated with an oxygen scavengersuch as CF₄, SnF₂ or PbF₂ to remove oxygen. When a metal powder is usedas the dopant, the scavenger treatment also converts the metal to metalions as well as removes oxygen. Similarly, the scavenger helps inremoving oxygen from metal oxide dopant thereby converting it to a metalfluoride.

The doped CaF₂ crystals used in the γ-ray tests described below weregrown using a crystal growth and annealing apparatus as described in the'461 patent. Summarizing, the apparatus as described in the ‘461 patenthas a primary heating system mounted near the top and sides of thecrystal and a secondary heating system mounted near the bottom of thecrystal. This secondary heating system may or may not be used during theproduction of the doped crystals. The method, generally, of the '461patent used to make the crystals described herein has steps of (1)forming a liquid of crystal material, including the dopant, in acrucible by heating the crystal material using heat from the primaryheating system; (2) lowering the crucible out of the primary heatingsystem so that successive portions of the liquid crystal material coolto a temperature suitable for crystal formation; (3) reducing thetemperature of the primary heating system; (4) raising the crucible intothe primary heating system and supplying heat from the secondary heatingsystem; and (5) reducing the heat output of the primary and secondaryheating systems so that the average temperature of the crystal is cooledover time. It is especially important to maintain a low temperaturegradient during the initial phases of cooling, when the hot crystal hasrelatively low yield strength. Cooling times in the range of 20-40 daysare described in the '461 patent. However, in the preferred case coolingtimes may be on the order of 10 to 25 days.

The growth of crystals of selected orientation, for example, a <111>,<110> or <100> crystal can be done by using a crucible having areservoir in its bottom, as illustrated in FIGS. 1 and 2, into which,for example, a <111> seed crystal is placed. After the doped CaF₂ hasbeen prepared it can be annealed to reduce stresses within the crystaland the birefringence that may result from such stress. Such annealingmethods have been described in the art; for example, in U.S. Pat. No.6,806,039.

The doped crystals of the invention can also be grown using the methoddescribed in the '039 patent. FIGS. 1-3 herein illustrate the some offeatures of the crystal growth process described in the '039 patent andbriefly summarized as follows. Lead fluoride was used as an oxygenscavenger.

FIG. 1 shows a crystal growth crucible 62 for growing doped crystalhaving a crystal growth chamber and a seed crystal orientation receiver64 for receiving and orienting a seed crystal 60 in relation to theadjoining above crystal growth chamber (designated herein 90). Arrow 92shows the preferred crystal axis direction of the seed crystal. FIG. 2shows the growth crucible loaded with the seed crystal 60 and the CaF₂feedstock 70 containing the selected dopants as described herein. In thepreferred case, a seed crystal may not be used during the crystal growthprocess. The optical crystal is later removed from the large bulkcrystal in a manner that provides an optical element whose surfaces havethe desired crystallographic orientation. The machining techniques usedto produce this optical element with the desired crystallographicsurface orientations are known in the art. FIG. 3 shows the crystalgrowth crucible 62, with lid 63 thereon, containing the doped feedstockas a melt 66 with an upper portion of seed crystal 60 melted. The dopedfeedstock was melted in the upper hot melt zone of controlled atmospherevacuum furnace 110. Controlled atmosphere/vacuum furnace 110 was heatedby resistive graphite heating elements 8. An insulating furnace baffle14 preferably separates the upper and lower heating elements to isolatethe lower cool anneal zone (below the baffle) from the upper hot meltzone (above the baffle) and forms there between a crystal growthtemperature gradient. The partially melted crystal seed 60 and melteddoped feedstock 66 is progressively moved through the crystal growthtemperature gradient to grow a seeded oriented doped CaF₂ crystal. Afterthe single crystal is fully grown it can be cooled as described hereinor elsewhere in the art within the lower portion of the growth furnaceor it can be cooled and moved to a separate annealing furnace accordingto the schedule given above or other annealing schedules known in theart.

It is recognized to those skilled in the art that the localconcentration of a specific dopant may vary axially throughout thecrystal. The degree of dopant variation is dependent upon thesegregation coefficient of the dopant within the material, the rate ofcrystal growth, the diffusivity of the dopant within the moltenmaterial, and the convective state of the molten material during growth.Careful measurements made using ICP-MS have been used to identify theamount of dopant present in the optical elements tested. It is theactual measured dopant concentration values which are discussed herein.

As stated above, it is known that polished but uncoated surfaces of CaF₂are susceptible to degradation when exposed to powerful lasers operatingin the DUV and VUV ranges. For example, when using 193 nm lasersoperating at 2-9 KHz with pulse densities of 20-80 mJ/cm², the surfacesor the optical elements made from these ionic materials are known tofail after only a few million laser pulses. R. Bennewitz et al, “Bulkand surface processes in low-energy-electron induced deposition of CaF₂”, Amer. Physical Society, Physical Review B, Vol. 59, No. 12 (1999),pages 8237-8246, suggest that the cause of the damage is thought to befluorine diffusion from the bulk of the crystal to the surface.Bennewitz et al indicate that metal (Ca) formation was observed on thesurface of the crystal and that “Colloid formation [in the crystal]results from aggregation of F centers, a process favored in CaF₂ by thegood match between the lattice structure and atomic spacing of calciummetal and the Ca²⁺ sublattice in CaF₂.” FIG. 5 shows the Raman spectrumof CaF₂ before and after exposure to 193 nm laser radiation. The changein the Raman spectra demonstrates the existence of Ca colloids in CaF₂after exposure spectrum to 193 nm laser radiation. U.S. Pat. No.6,466,365 (the '365 patent) describes a method of protecting metalfluoride surfaces, such as CaF₂, from degradation by use of a vacuumdeposition, of a silicon oxyfluoride coating/material. While for themoment this is a reasonable solution, the microlithographic industryconstantly demands more performance from excimer sources, andconsequently from optical components used in connection with excimerlaser based systems. In particular, the industry would prefer to useuncoated CaF₂ optics because of the reduced costs, better transmission,and the general outlook that the less complex the optic, the less likelyit is that something will go wrong. The lithographic industry iscurrently seeking optics that can survive as many as 50 billion pulsesof 20-80 mJ/cm² with an acceptably low level of degradation over thisperiod. Coating the optics, by itself, is believed insufficient to reachthis goal without improvements in the laser durability of the bulkmaterial.

Disclosed herein are optics made of single crystal CaF₂ doped with oneor more dopant materials in specific amounts selected from the groupconsisting of Mg, Sr and Ba (“dopant”) in order to extend the lifetimeof the CaF₂ optic when it is used in high power laser systems; forexample, lasers operating at 193 nm, 2-9 KHz, with pulse energydensities of 20-80 mJ/cm². The amount of each dopant selected to add toCaF₂ is from within the following ranges; >0.3-1200 ppm Mg, >0.3-200 ppmSr. and >0.3-200 ppm Ba. Each of these dopants form solid solutions withCaF₂ within the given concentration ranges. Each dopant also has anatomic radius that differs from the Ca ion within the crystal lattice.The ionic radii values (Pauli, in Angstroms) are Mg=0.69, Ca=0.99,Sr=1.13 and Ba=1.45. This difference in atomic radii distorts thecrystal lattice in a manner that reduces the time required for therecombination of excitons created with the CaF₂ structure by exposure tolaser irradiation. While the addition of one or more dopants reduces theexciton lifetime, it does not prevent the formation of all latticedefects caused by exposure to radiation. However, the addition of one ormore dopants does appear to inhibit the formation of Ca colloids thatare typically associated with laser damage in CaF₂ single crystals

In one embodiment, the present invention is directed to an alkalineearth crystal consisting of CaF₂ as the major component and at least onedopant selected from the group consisting of >0.3-1200 ppm Mg, >0.3-200ppm Sr, >0.3-200 ppm Ba. In another embodiment the dopants are selectedfrom the group consisting of Ce and Mn in an amount of less than <0.5ppm of the selected dopant. In another embodiment the alkaline earthsingle crystal consists of CaF₂ as the major component and at least onedopant selected from the group consisting of >2-500 ppm Mg, >2-100 ppmSr, >2-100 ppm Ba. In a further embodiment the invention consists ofCaF₂ as the major component and at least one dopant selected from thegroup consisting of >10-100 ppm Mg, 5-50 ppm Sr, >2-10 ppm Ba. In anadditional embodiment the alkaline earth single crystal consists of CaF₂as the major component and at least one dopant selected from the groupconsisting of >20-100 ppm Mg, 1.0-200 ppm Sr, and >1.0-200 ppm Ba. In afurther embodiment CaF₂ is the major component and the dopant is 20-60ppm Mg.

Mixed alkaline earth metal fluorides have been described in the both thepatent and technical literature. For example, U.S. Pat. Nos. 6,806,039,6,630,117, 6,649,326, and U.S. Patent Publication No. 2003/0104318,describe making mixed alkaline earth fluoride single crystals of generalformula M¹ _(x)M² _((1-x))F₂ where x is in the range of 0.1-0.9; suchmixed metal crystals all containing greater than 10,000 ppm of thelesser of the two alkaline earth metal ions. V. Denks et al., “Excitonicprocesses in pure and doped CaF ₂,” J, Phys. Condens. Matter, Vol. 11(1999), pages 3115-3125, investigated CaF₂ doped with Mg, Mn, Na and Liions. The authors investigated CaF₂ crystals doped with (a) Mg ions inamounts in the range of 0.01-0.1% (page 3117) or 0.2% Mn ions (page3119). In their conclusion on page 3124, regarding impurities [dopants],they stated “None of the impurities (Mg or Mn) described in the presentpaper led to an improvement of the radiation stability of CaF₂.” Thisconclusion was based upon their fluorescence measurements and iscontrary to concepts and information put forth herein. In addition,Denks et al. state, without specification, that they did find animpurity which might raise the radiation resistance of CaF₂. In asubsequent paper, V. Denks et al., “Impurity-Related Excitonic Processesin CaF ₂-Sr”, Phys, Stat. Sol. (a), Vol. 191. No. 2, (2002), pp. 628-632describes a CaF₂:Sr single crystals in which Sr ranges from 0.05 to 4mol % (0.05 mol %=˜561 ppm or 0.6 wt % Sr). In this subsequent paper,Denks et al. conclude that doping CaF₂ with Sr at this high level mayimpart increased durability to radiation exposure. In some patents, forexample, U.S. Pat. No. 6,999,408, Mg, Sr and Ba were regarded asimpurities in CaF₂ and were kept to level below 0.5 ppm Mg, 19 ppm Srand 5 ppm Ba. Neither do these patents recognize the ability of thesespecific metallic ions at specific dopant levels to impart increasedlaser durability to CaF₂.

It is also highly desirable to have an accelerated test by which dopedsingle crystal CaF₂ optics can be laser durability tested. Presently,the accelerated test methods use a very high power excimer laser and canlast anywhere from a few days to several weeks. This method of testingis both expensive and time consuming Other methods (for example, laserfluorescence as cited above in Denks et al.) have been investigated todetermine whether they could accurately indicate a CaF₂ optic's laserdurability; however, these methods have only met with limited success.Currently, the only viable method to “rapidly” evaluate the improvedlaser durability of doped CaF₂ optical elements was suggested by T. D.Henson et al. in “Space radiation testing of radiations resistantglasses and crystals”, Proc, SPIE. V4452 (1001), pp 54-65. Henson et al.suggest that transmission testing after exposure to γ-radiation servesas a viable test method of the durability of CaF₂ optics. Therefore,this method was employed to evaluate doped CaF₂ samples as described inthis disclosure. Samples of doped and un-doped CaF₂ optics having athickness of 7 mm were exposed to a dose of between 28.3 and 28.7 kGy(2.83-2.87 MRad) using a gamma-ray [γ-ray] source. The transmissionspectra from 200 to 1000 nm of the samples were tested before exposureand again at 25, 100, 430 and 600 hours after γ-radiation exposure. Itwas found that the doped CaF₂ crystals with improved laser durabilityhad a lower ratio of 515/380 transmission loss than undoped CaF₂material had. The 515/380 transmission loss ratio is defined as thedecrease in transmission at 515 nm after exposure compared to beforeexposure divided by the similar loss in 380 nm transmission afterexposure compared to before exposure. These particular wavelengths arecompared because the presence of Ca colloids results in absorption ataround 515 nm while F center presence results in absorption at around380 nm (an F center is a fluoride ion vacancy with one electron is inthe vacancy). During the course of the evaluation of the irradiateddoped and undoped (D and UD) samples, it was found that while both the Dand UD samples have F centers (decreased 380 nm transmission), the Dsamples do not appear to making colloids whereas the UD do make colloids(decreased 515 transmission). This result is particularly impressivesince the precursor to colloid formation is the presence of F centers.Apparently, at low concentrations of a dopant such as Mg as used in thepresent sample optics, the dopant inhibits colloid formation which inturn improves laser lifetime.

Generally, it was found that un-doped CaF₂ optic samples (UD) had a lossratio after exposure greater than 0.4 and that the ratio increased onthe order of 25% as transmission recovery after exposure increased,though the increase was at a gradually leveling-out rate. In contrast,the doped CaF₂ optic samples (D) has a loss ratio of less than 0.3throughout the entire evaluation period indicating less colloidformation for given amount of F center formation. In some embodimentsthe loss ratio of the D optic samples was less then 0.2. In the exampleshown in FIG. 4 the loss ratio was less than or equal to 0.1. The Doptics contained were Mg-doped in the range of 10-100 ppm, preferably inthe range of 20-80 ppm.

Thus, in one embodiment the invention is directed to a laser opticcomprising a CaF₂ crystal material doped with a selected amount of aselected dopant whose purpose is to inhibit the formation of Ca colloidsand thereby impart improved laser durability to the optical element. Thepurpose of the selected dopant is to inhibit the formation of Cacolloids and thereby impart improved laser durability to the opticalelement. In one embodiment the colloid inhibiting dopant and amount isone selected from the group consisting of >0.3-1200 ppm Mg, >0.3-200 ppmSr, >0.3-200 ppm Ba is added to inhibit the formation of Ca colloids. Inanother embodiment the colloid inhibiting dopant is Mg in an amount inthe range of 2-500 ppm. In further embodiment the colloid inhibitingdopant is Mg in an amount in the range of 10-100 ppm. The foregoinglaser optics have a ratio of 515/380 nm transmission loss of less than0.3 after exposure to greater than 2.8 MRads of γ-radiation. In oneembodiment the ratio of 515/380 nm transmission loss of less than 0.2after exposure to greater than 2.8 MRads of γ-radiation. In anotherembodiment the ratio of 515/380 nm transmission loss of less than orequal to 0.1 after exposure to greater than 2.8 MRads of γ-radiation.

The doped CaF₂ optics according to the invention can be coated oruncoated. The coating materials can be a materials selected from thegroups consisting of fluoride, oxide and fluorinated oxide films whichare applied to the surfaces of the optic using advanced plasmatechniques known in the art. Example of such coating materials and thetechniques for coating optics can be found in the commonly-owned U.S.Pat. No. 7,242,843 and citations therein whose teachings areincorporated by reference. The coating material can be applied directlyto the optic. Coating materials include SiO₂.F, Al₂O₃, MgF₂, BaF₂, CaF₂,SrF₂, NaF, LiF, AlF₂, LaF₃, GdF₃, NdF₃, DyF₃, YF₃ and ScF₃. The opticsto be coated include prisms, windows and lenses, and can further includemirrors made of CaF₂.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover modifications and variationsof this invention provided they come within the scope of the appendedclaims and their equivalents.

1. A laser optic having improved laser durability, said optic comprisinga CaF₂ crystal material doped with a selected amount of a selecteddopant, and said optic having a ratio of 515/380 nm transmission loss ofless than 0.3 after exposure to greater than 2.8 MRads of γ-radiation.2. The laser optic according to claim 1, wherein the dopant and amountis one selected from the group consisting of >0.3-1200 ppm Mg, >0.3-200ppm Sr, >0.3-200 ppm Ba.
 3. The laser optic according to claim 1,wherein the dopant and amount is 2-500 ppm Mg.
 4. The laser opticaccording to claim 1, wherein the dopant and amount is 10-100 ppm Mg. 5.The laser optic according to claim 1, wherein the ratio of 515/380 nmtransmission loss is less than 0.2 after exposure to greater than 2.8MRads of γ-radiation.
 6. The laser optic according to claim 1, whereinthe ratio of 515/380 nm transmission loss of less or equal to 0.1 afterexposure to greater than 2.8 MRads of γ-radiation.
 7. A laser optichaving improved excimer laser durability, said optic comprising a CaF₂single crystal material doped with 20-100 ppm Mg, and optic having aratio of 515/380 nm transmission loss of less than 0.2 after exposure togreater than 2.8 MRads of γ-radiation.
 8. The laser optic according toclaim 7 , wherein said optic has a coating thereon, said coating beingat least one material selected the group consisting of SiO₂.F, Al₂O₃,MgF₂, BaF₂, CaF₂, SrF₂, NaF, LiF, AlF₃, LaF₃, GdF₃, NdF₃, DyF₃, YF₃ andScF₃.
 9. A laser optic having improved laser durability, said opticcomprising a CaF₂ single crystal material doped with 20-100 ppm Mg, andoptic having a ratio of 515/380 nm transmission loss of less than 0.2after exposure to greater than 2.8 MRads of γ-radiation.
 10. The laseroptic according to claim 9, wherein the ratio of 515/380 nm transmissionloss of less than or equal to 0.1 after exposure to greater than 2.8MRads of γ-radiation.
 11. The laser optic according to claim 9, whereinsaid optic has a coating thereon, said coating being at least onematerial selected the group consisting of SiO₂.F, Al₂O₃, MgF₂, BaF₂,CaF₂, SrF₂, NaF, LiF, AlF₃, LaF₃, GdF₃, NdF₃, DyF₃, YF₃ and ScF₃.
 12. Adoped CaF₂ crystal suitable for making laser optics having improvedlaser durability, said crystal consisting of CaF₂ as the majorcomponent, and at least one dopant selected from the group consisting ofthe group consisting of >0.3-1200 ppm Mg, >0.3-200 ppm Sr, >0.3-200 ppmBa
 13. The doped crystal according to claim 12, wherein the dopant andamount, measured as metal ion, is 2-500 ppm Mg.
 14. The doped crystalaccording to claim 12, wherein the dopant and amount, measured as metalion, is 10-100 ppm Mg.
 15. The doped crystal according to claim 12,wherein said crystal has a ratio of 515/380 nm transmission loss of lessthan 0.3 after exposure to greater than 2.8 MRads of γ-radiation. 16.The doped crystal according to claim 12, wherein said crystal has aratio of 515/380 nm transmission loss of less than 0.2 after exposure togreater than 2.8 MRads of γ-radiation.
 17. The doped crystal accordingto claim 12, wherein said crystal has a ratio of 515/380 nm transmissionloss of less than or equal to 0.1 after exposure to greater than 2.8MRads of γ-radiation.
 18. A laser optic, said optic comprising a CaF₂crystal material doped with a selected amount of a selected dopant whosepurpose is to inhibit the formation of Ca colloids and thereby impartimproved laser durability to the optical element.
 19. The laser opticaccording to claim 18, wherein the colloid inhibiting dopant is oneselected from the group consisting of >0.3-1200 ppm Mg, >0.3-200 ppmSr, >0.3-200 ppm Ba is added to inhibit the formation of Ca colloids.20. The laser optic according to claim 18, wherein the colloidinhibiting dopant is one selected from the group consisting of 10-100ppm Mg.
 21. The laser optic according to claim 18, wherein said optichas a ratio of 515/380 nm transmission loss of less than 0.3 afterexposure to greater than 2.8 MRads of γ-radiation.